Dissipation of heat generated by equipment depends on many factors, several of which are not compensated for by present cooling systems. The heat dissipation capacity of a fluid depends on the ambient temperature of the fluid, its rate of flow past the object to be cooled, its density, and the specific heat of the fluid. Decreasing the density or flow or increasing the temperature significantly reduces the cooling effectiveness of the medium.
It is desirable to maintain a constant level of cooling, that is, to remove the same amount of heat as the object to be cooled produces at a constant operating temperature. However, most present systems simply cool equipment at a single, constant rate of flow or rate of heat extraction, that is, refrigeration: if the heat dissipation capacity of the cooling fluid decreases, the equipment eventually overheats and its components may become damaged even if the equipment continually generates the same amount of heat. Heat dissipation capacity of a fluid is decreased, for example, when its temperature increases or its density decreases without a corresponding increase in flow. Further, an increase in operating temperature may exceed the fixed flow or refrigeration level.
There a number of applications in which it is preferable to cool buildings and equipment accurately, that is, to cool them efficiently using only as much forced air movement or refrigeration as actually required. Fluidflow systems, e.g., systems utilizing forced air moved by fans, must have sufficient capacity to adequately cool the equipment yet are frequently quite noisy especially at maximum cooling rates. Use of sophisticated office equipment which require cooling is increasing dramatically, but the attendant noise in the office environment must be maintained within acceptable bounds. In addition to producing objectionable noise at maximum cooling rates, equipment is increasingly used in a variety of operating environments which range from sea-level to high altitudes, and from temperate climates to desert temperatures. An increase in altitude decreases the density of the cooling air and decreases its heat dissipation capacity, as does an increase in temperature of the air.
One system for measuring the cooling effectiveness of a fluid utilizes two thermometers, one positioned to measure the ambient temperature and the other positioned at the exit of the cooling medium from the machine. A rise in the exit temperature indicates that the cooling effectiveness of the medium is insufficient. However, the difference in exit temperature between best case cooling and worst case cooling may be quite small, e.g., only 5.degree.-6.degree. C. Consequently, accurate control of the fluid mover, e.g., a fan, is difficult at present.
Moreover, when there is a loss of cooling air in equipment such as a computer, the entire flow field, that is, the flow pattern of the air, often changes dramatically. The flow pattern may even become reversed. When this occurs, the temperature at the sensing point may bear no relationship to the actual temperature of one or more portions of a computer. Once this relationship is lost, the monitoring system is no longer reliable and serious overheating of components often results.
There are several systems which measure the resistance of a sensor rather than directly measure temperature with a thermometer. Any material exhibits a change in electrical resistance when its temperature increases; typically, resistance increases with increasing temperature. As described in U.S. Pat. No. 4,476,720 by Ismail et al., some systems monitor a thermistor in an air flow whose resistance lowers when temperature rises. Reduction in resistance to a preselected value is simply detected as an alarm condition. The systems do not modify the heat dissipation capacity of the air flow, e.g., by selectively varying the flow of the air or its temperature.
In hot-wire anemometry, change in resistance of a wire is used to measure velocity. Current is passed through a wire and control circuitry maintains the wire at a constant temperature; alternatively, the circuitry applies a constant level of current to the wire. The amount of cooling experienced by the wire is used to calculate the velocity of the fluid in which it is placed. Temperature compensating circuits eliminate the effect of ambient temperature so that the hot-wire device functions solely as a velocity sensor.